The present invention relates broadly to form-in-place (FIP) electromagnetic interference (EMI) shielding gaskets or seals which are formed and cured in place under atmospheric pressure on the surface of a substrate, and particularly to a FIP method of making a low closure force FIP gasket which is particularly adapted for use within small electronics enclosures such as cellular phone handsets and other handheld electronic devices.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed xe2x80x9celectromagnetic interferencexe2x80x9d or xe2x80x9cEMIxe2x80x9d as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other xe2x80x9ctargetxe2x80x9d devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. For further information on specifications for EMI shielding gaskets, reference may be had to Severinsen, J., xe2x80x9cGaskets That Block EMI,xe2x80x9d Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).
EMI shielding gaskets typically are constructed as a resilient core element having gap-filling capabilities which is either filled, sheathed, or coated with an electrically conductive element. The resilient core element, which may be foamed or unfoamed, solid or tubular, typically is formed of an elastomeric thermoplastic material such as polyethylene, polypropylene, polyvinyl chloride, or a polypropylene-EPDM blend, or a thermoplastic or thermosetting rubber such as a butadiene, styrene-butadiene, nitrile, chlorosulfonate, neoprene, urethane, or silicone rubber.
Conductive materials for the filler, sheathing, or coating include metal or metal-plated particles, fabrics, meshes, and fibers. Preferred metals include copper, nickel, silver, aluminum, tin or an alloy such as Monel, with preferred fibers and fabrics including natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, polyimide. Alternatively, other conductive particles and fibers such as carbon, graphite, or a conductive polymer material may be substituted.
Conventional manufacturing processes for EMI shielding gaskets include extrusion, molding, or die-cutting, with molding or die-cutting heretofore being preferred for particularly small or complex shielding configurations. In this regard, die-cutting involves the forming of the gasket from a cured sheet of an electrically-conductive elastomer which is cut or stamped using a die or the like into the desired configuration. Molding, in turn, involves the compression or injection molding of an uncured or thermoplastic elastomer into the desired configuration.
More recently, a form-in-place (FIP) process has been proposed for the manufacture of EMI shielding gaskets. As is described in commonly-assigned, co-pending application U.S. Ser. No. 08/967,986, filed Nov. 12, 1997; U.S. Pat. Nos. 5,910,524 and 5,641,438; European Patent Applications EP 643,551 and 643,552; and PCT Applications WO/9622672 and WO/9507603; and in U.S. Pat. Nos. 5,882,729 and 5,731,541; and Japanese Patent Publication (Kokai) No. 7177/1993, such process involves the application of a bead of a viscous, curable, electrically-conductive composition which is dispensed in a fluent state from a nozzle directly onto to a surface of a substrate such as a housing or other enclosure. The composition, typically a silver-filled or otherwise electrically-conductive silicone elastomer, then is cured-in-place via the application of heat or with atmospheric moisture or ultraviolet (UV) radiation to form an electrically-conductive, elastomeric EMI shielding gasket in situ on the substrate surface. By forming and curing the gasket in place directly on the substrate surface, the need for separate forming and installation steps is obviated. Moreover, the gasket may be adhered directly to the surface of the substrate to further obviate the need for a separate adhesive component or other means of attachment of the gasket to the substrate. In contrast to more conventional die cutting or molding processes, the flashless FIP process reduces waste generation, and additionally is less labor intensive in that the need for hand assembly of complex gasket shapes or the mounting of the gasket into place is obviated. The process, which is marketed commercially under the name CHO-FORM(copyright) by the Chomerics Division of Parker-Hannifin Corp., Woburn, Mass., further is amenable to an automated or roboticly-controlled operation, and may be employed to fabricate complex gasket geometries under atmospheric pressure and without the use of a mold.
As the above-described FIP process continues to garner commercial acceptance, it will be appreciated that further improvements in this process and in materials therefor would be well-received by the electronics industry. In this regard, certain applications specify a low impedance, low profile gasket which is deflectable under relatively low closure force loads, e.g., about 0.4-10 Newton per centimeter of gasket length. Generally, a minimum deflection, typically of about 10%, is specified to ensure that the gasket sufficiently conforms to the mating housing or board surfaces to develop an electrically conductive pathway therebetween. It has been observed that for certain applications, however, that the closure or other deflection force required to effect the specified minimum deflection of the FIP gasket profiles heretofore known in the art may be higher than can be accommodated by the particular housing or board assembly design. Thus, it will be appreciated that further improvements in the manufacture of FIP gaskets profiles would be well-received by the electronics industry. As the sizes of handheld electronic devices such as cellular phone handsets has continued to shrink, especially desired therefore would be a low closure force FIP gasket profile which is especially adapted for use in the smaller electronics enclosures which are rapidly becoming the industry standard.
Broad Statement of the Invention
The present invention is directed to the FIP manufacture of a low closure force EMI shielding spacer gasket profile especially adapted for use in smaller electronic enclosure packages. In having a periodic xe2x80x9cinterruptedxe2x80x9d pattern of alternating local maxima and minima bead heights, the gasket profile of the present invention is seen to exhibit lower closure force requirements than the FIP gasket profiles heretofore known in the art. That is, for a specified joint configuration, the spacer gasket profile of the present invention exhibits a greater deflection under a given compressive load than conventional profiles.
Conventionally, and as is described further in commonly-assigned, co-pending application U.S. Ser. No. 09/042,135, filed Mar. 13, 1998, in the Technical Publication, xe2x80x9cEMI Shielding and Grounding Spacer Gasket,xe2x80x9d Parker Chomerics Division, Woburn, Mass. (1996), and in PCT application 98/54942, the xe2x80x9cinterruptedxe2x80x9d EMI shielding gaskets of the type herein involved are formed principally by molding. The present invention, however, provides for the manufacture of these gaskets as a series of FIP beads which are dispensed as a fluent composition from a nozzle directly onto to a surface of a substrate such as a housing or other enclosure, and then cured in situ under atmospheric pressure on the surface of a substrate. The composition may be provided as a filled silicone elastomer which is curable in place, such as via the application of heat or with atmospheric moisture or ultraviolet (UV) radiation, to form an electrically-conductive, elastomeric EMI shielding gasket in situ on the substrate surface. Advantageously, by forming and curing the gasket in place directly on the substrate surface, the need for separate forming and installation steps is obviated. Moreover, the gasket may be adhered directly to the surface of the substrate to further obviate the need for a separate adhesive component or other means of attachment of the gasket to the substrate.
The present invention, therefore, comprehends the formation of a low closure force gasket having alternating high and low contact points. The gasket is formed-in-place (FIP) on a surface of a substrate as a bead of a curable elastomeric composition which is issued under an applied pressure from the orifice of a nozzle. The is nozzle is movable relative to the surface of the substrate along at least a first axis disposed generally parallel to the substrate surface, and, optionally, along a second axis disposed generally perpendicular to the substrate surface. The nozzle is moved at a predetermined speed along the first axis, and, optionally, along the second axis to apply the bead along a given path on the substrate. One or more of the applied pressure, the speed of movement of the nozzle along the first axis of step, and the movement of the nozzle along the second axis are controlled to apply the bead in a periodic series of alternating high and low intervals relative to the substrate surface. The elastomeric composition then is cured under substantially atmospheric pressure to form the gasket on the substrate surface, with the high intervals of the bead defining the high contact points of the gasket, and with the low intervals of the bead defining the low contact portions of the gasket.
In one disclosed embodiment the movement of the nozzle along the second axis is controlled by reciprocating the nozzle intermediate an upper and a lower position relative to the substrate surface. Such movement defines with the movement of the nozzle along the first axis a generally sinusoidal motion of the nozzle within a plane disposed transverse to the substrate surface. Through such motion, the bead may be applied to the substrate surface as having a generally continuous wavefrom profile of alternating peaks and troughs, with the peaks defining the high intervals of the bead and the troughs defining the low intervals of the bead.
In another disclosed embodiment, the speed of movement of the nozzle along the first axis is controlled by defining a series of spaced-apart points along the path. As the nozzle approaches each of these points, the speed of the nozzle is decreased and, thereafter, accelerated as the nozzle travels intermediate each of the points.
In another disclosed embodiment, the applied pressure is periodically increased and decreased as the nozzle is moved along the first axis.
In yet another disclosed embodiment, the applied pressure and the speed of movement of the nozzle along the first axis both are controlled by defining a series of spaced-apart points along the path. The movement of the nozzle along the first axis is stopped at each of these points for a predetermined dwell period with pressure being applied to issue the bead from the nozzle. Thereupon, the nozzle is moved intermediate each of the points with the application of the pressure being discontinued to stop the bead from issuing from the nozzle. Through such control, the bead may be applied to the substrate surface in a pattern of discrete dots which define the high intervals of the bead. The dots are separated by spaces which define the low intervals of the bead.
Advantages of the present invention include the manufacture of a gasket profile for low closure force applications such as may be found in small, handheld electronic devices. Additional advantages include a method of manufacture by which such gasket profile may be formed-in-placed via the application of a bead of a viscous, curable, electrically-conductive composition which is dispensed in a fluent state from a nozzle directly onto to a surface of a substrate such as a housing or other enclosure. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.